You need to measure the SPL from the earpiece (HP, iem, etc). No any other measurement system can give you correct-healthy results. Any Android, IOS or Windows Mobile phone can take measurements by the help of an Application. When I am away from my professional Sound Pressure Level meter I am using SmartTools for Android APP. There is a decibel meter in the APP. You just get the phone's microphone very close enough to the earpiece of an headphone or push the iem silicone eartip on the mic (totally covering the mic) and get the reading (while playing music, in a quiet place). Not a scientific method or extremely accurate results but better not having any idea. I have compared these results to my scientific grade SPL meter and differences were not very huge. So better download the app and start measuring!